theorem
  i1,i2 are_congruent_mod i5 & i3,i4 are_congruent_mod i5 implies
  (i1 - i3),(i2 - i4) are_congruent_mod i5
proof
  assume that
A1: i1,i2 are_congruent_mod i5 and
A2: i3,i4 are_congruent_mod i5;
  consider i8 such that
A3: i5 * i8 = i1 - i2 by A1;
  consider i9 such that
A4: i5 * i9 = i3 - i4 by A2;
  (i1 - i3) - (i2 - i4) = i5 * (i8 - i9) by A3,A4;
  hence thesis;
end;
